1. Field of the Invention
This invention relates to a color image forming apparatus adapted to form a toner image on a belt-shaped image retainer by an electrophotographic system, to transfer the toner image on a transfer material to obtain an object image.
2. Description of the Prior Art
Many methods and apparatuses for obtaining color images by using electrophotography have been proposed. As disclosed in, for example, Japanese Patent Laid-open No. 100770/1986, there is a method of obtaining a color copy by forming a latent image on a photosensitive drum serving as an image retainer in accordance with a color separation number of a document image, developing the latent image on the drum, transferring the resultant image onto a transfer drum every time the development of the latent image is completed to form a multi-color image on the transfer drum, and thereafter transferring the multi-color image onto a transfer material to obtain an object color copy. An apparatus realizing this method needs to be provided with a large transfer drum where a one-frame image is transferred on the circumferential surface thereof in addition to a photosensitive drum. Consequently, it is unavoidable that this apparatus has a large and complicated structure.
There is another method disclosed in, for example, Japanese Patent Laid-open No. 149972/1986, which comprises the steps of forming a latent image on a photosensitive drum in accordance with the color separation number of a document image, developing the latent images on the drum, and transferring the resultant image onto a transfer material every time the development of the latent image is completed, to obtain a multi-color copy. In this method, it is difficult to accurately superpose each of monochromatic images one upon another, so that a high-quality multi-color copy cannot be obtained.
There is also another method of obtaining a color image, which comprises the steps of repeating formation of a latent image on a photosensitive drum in accordance with the color separation number of a document image and development of the latent images with color toners, superposing color toner images one upon another on the photosensitive drum, and then transferring the resultant image onto a transfer material. The basic processes for formation of such a multi-color image are disclosed, for example, in Japanese Patent Laid-open Nos. 75850/1985, 76766/1985, 95456/1985, 95458/1985 and 158475/1985.
In a multi-color image forming apparatus for obtaining a color image by such an image superposing step, a plurality of developing devices storing different color toners therein are arranged around a photosensitive drum, and a latent image on this drum is developed by rotating the same drum generally a plurality of times to obtain a color image.
In addition to the photosensitive drum which has a photoconductive material applied or evaporated on the circumferential surface of the drum as described above, a belt-shaped image retainer which has a photoconductive material applied or attached to a flexible belt has also been proposed.
The belt-shaped image retainer(photosensitive belt) is formed by training the image retainer around rollers including a drive roller. Thus, this design is advantageous when constructing a color image forming apparatus in a compact form by utilizing the space efficiently. Also, because the photosensitive belt can run along a curve with a sharp curvature, a small diameter driving roller can be employed to utilize this curvature for separating the transfer material to prevent a defective separation thereof.
In a color image forming apparatus using the photosensitive belt, image forming means comprising a charging means, an image exposing means, and a plurality of developing devices, are provided on the circumference of said photosensitive belt in such a way that these image forming means face the photosensitive belt with a given space therebetween.
In order to allow the image forming means to face the photosensitive belt while keeping the given space therebetween, back up rollers are employed. However, this method of using back up rollers needs as many rollers corresponding to the image forming means. If such a large number of rollers are employed, difficulties are encountered in maintaining the parallelism with said driving roller. Also, there is proposed an apparatus, such as disclosed in Japanese Patent Laid-open No. 34576/1982, in which the image forming means is positioned to face the photosensitive belt running on the guide members. However, it is difficult for an apparatus, such as this, to provide a given space required.
In the case of a photosensitive drum, a rotating roller functioning as a space retaining member is mounted coaxially with a developing roller in the developing device and the developing device is in contact with the photosensitive drum under pressure. However, in the case of a photosensitive belt, if the rotating roller is used as a means to retain a given space, the load applied to the photosensitive belt is increased when the roller is in contact therewith and slippage and other undesirable operations are generated.